According to the related art, techniques for detecting the amount of displacement of a moving body in linear movement have been used. Examples of such techniques include the following JP 2002-139301A (Reference 1) and JP 2009-42154A (Reference 2).
A differential transformer-type displacement sensor that is described in Reference 1 is configured for an induced electromotive force to be generated in two secondary coils by an alternating current flowing in a primary coil. The induced electromotive force that is generated in the two secondary coils changes depending on displacement of a magnetic body core which is capable of reciprocating, and the differential transformer detects the amount of movement of the magnetic body core from a potential fluctuation.
A position sensor that is described in Reference 2 is configured to be provided with a reciprocating movable yoke, a ring-shaped fixed yoke that is disposed in a state of being inserted into the movable yoke, and a magnetic sensor that is arranged between two magnets which are arranged in parallel to the movable yoke. No magnetic flux passes through the magnetic sensor when the movable yoke is positioned at the center. The magnetic flux passes through the magnetic sensor when the movable yoke is displaced from the center. The position sensor detects the amount of displacement by detecting a magnetic flux density through the magnetic sensor.
According to the technique that is described in Reference 1, an auxiliary coil is used so as to improve detection sensitivity in an end portion in a direction of movement. However, a range of coil arrangement has to be wider than a range of movement of the magnetic body core, and thus an axial-direction length of the coil increases to pose a mountability problem. According to the technique that is described in Reference 2, the magnet is arranged in parallel to the direction in which the movable yoke can be moved, and the density of the magnetic flux that passes through the magnetic sensor is affected by a distance between the yoke and the magnet. In addition, the magnetic flux density that is detected by the magnetic sensor becomes erroneous when the movable yoke causes an axial misalignment vertical to a direction of the displacement. Accordingly, an output fluctuation increases with respect to the axial misalignment of the movable yoke. According to the technique that is described in Reference 2, a combined length of the two magnets and the magnetic sensor is equal to or larger than the amount of displacement. This poses a mountability problem in the sensor as is the case with the technique that is described in Reference 1.